I have the Dragonfly front wheel drive handcycle attachment from http://riomobility.com/en/
and it is wonderful. One problem with all front wheel drive machines is the lack of traction when going uphill, some hills cannot be be climbed. It's worse when it's wet. For steep hills rear wheel drive is the only practicable solution I can think of.
I have very little use of one arm and about half strength in the other. Going backwards is very difficult in the handcycle and a reverse gear would be an excellent option. I came across the Sturmey Archer QS-RC5(W) 5-speed gear box at http://www.sturmey-archer.com/
which has a coaster brake and a 1:1 reverse gear. It's designed as a gear box with one chain in and one chain out and the manufacturer said it couldn't be built into a wheel. It is built around a standard hub and I got someone to thread it into a wheel and it works great. There are some issues with it but it is wonderful.
Because I have limited strength I need a very wide range of gears if I am to get up hills and still have a decent gear on the flat. I installed a Mountain Drive from http://www.schlumpf.ch/
which has a 1:1 standard gear and a 2.5:1 low gear. Combined with my 256% geared hub it gives me a total gear range of 640% which is massive. It also gives me 2 speeds for reverse.
has dozens of links for handcycles.